Linearization of a Radio Frequency (RF) amplifier is an important design technique to achieve a high linearity of an RF amplifier. The term “linearity” refers to the ability of the amplifier to produce signals that are accurate copies of the input but at increased power levels. In an ideal RF amplifier, the transfer function that describes the relationship of input power to output power is linear. In actual RF amplifiers, the transfer function is a non-linear function. Typically, as the input power increases, the amplifier has more and more difficulty maintaining the same level of amplification, i.e., the gain begins to drop. When the output of the amplifier differs from the input, this difference introduces distortion into the output. For example, when the output of an amplifier is clipped, this introduces high-frequency components into the output signal that were not present in the input signal. Thus, in real (not simulated or ideal) RF power amplifiers, the gain is a non-linear function that varies with input signal level. However, in real RF power amplifiers, the gain varies due to “memory effects,” which cause the gain to vary not only with input signal level, but also as a function of other parameters such as frequency, temperature, etc.
There are several conventional approaches to improving linearity of an RF amplifier. In one approach, a diode is used to introduce predistortion compensation so that the predistortion produced by the diode and the distortion produced by the RF amplifier cancel each other out. Another approach is to use a second harmonic short circuit and/or video band short circuit (i.e., a capacitor) to reduce the second harmonic impedance and reduce the memory effect [1, 2, 3, 4, 5, 6], where “video band” refers to the frequency range between 0 and 100 MHz. Yet another technique is to use an impedance tuner at the RF frequency domain to improve the linearity [7]. This technique involves manipulation of the signal at the carrier (e.g., RF) frequency rather than at the data (e.g., video) frequency.